The present invention relates to a waste water treatment method capable of treating fluorine waste water containing organic matter (in particular, surface active agent having low biodegradability), phosphor and hydrogen peroxide discharged from a semiconductor plant, a liquid crystal plant or the like while producing a small amount of waste as sludge, saving energy and reusing chemicals, and the invention also relates to waste water treatment equipment implementing this waste water treatment method.
In a semiconductor plant, a liquid crystal plant or the like, a surface active agent is used while being mixed in a variety of chemicals used by the manufacturing apparatus. In the manufacturing apparatus of the semiconductor plant or the like, it is a general practice to use phosphoric acid, hydrogen peroxide and hydrofluoric acid, and among others, there is a tendency that a large amount of hydrofluoric acid is used. For the above reasons, fluorine waste water containing a surface active agent, phosphor and hydrogen peroxide is discharged from the semiconductor plant, the liquid crystal plant or the like.
In this case, the aforementioned "fluorine waste water containing a surface active agent, phosphor and hydrogen peroxide" means an "acid waste water that mainly includes fluorine and contains a small amount of surface active agent, phosphor and hydrogen peroxide" and further means a "hydrofluoric acid waste water that mainly includes fluorine and contains a small amount of surface active agent, phosphor and hydrogen peroxide".
The mixture of the surface active agent in the various chemicals in the semiconductor plant, the liquid crystal plant or the like is for the reasons as follows. That is, under the circumstances of the rapid microstructural progress of semiconductor devices, elaborate portions cannot be sufficiently cleaned by only the aqueous cleaning with ultrapure water due to the surface tension of the water. Therefore, a method for cleaning the elaborate portions by reducing the surface tension with the mixture of a surface active agent in the ultrapure water and a method for executing the cleaning by reducing the surface tension with the mixture of a surface active agent (for example, a chemical such as buffered hydrofluoric acid containing a surface active agent) in a variety of cleaning chemicals are gaining popularity. In such a case, the types of the various surface active agents to be mixed in the chemicals are the important know-how of the chemical manufacturers, and newly developed surface active agents are consistently mixed.
Among the newly developed surface active agents, there are ones that have low biodegradability from the viewpoint of molecular formula, structural formula, effervescence, sterilizing performance and so on. This means that the conventional activated sludge method, catalytic oxidation method or the like as a representative of the biotic treatment, the methods become incapable of coping with the surface active agents since the methods have a microorganic concentration of about 2000 ppm to 5000 ppm.
According to a recent report, there is the indication that some surface active agents become hormone disrupters, and some countermeasures are required to be taken. On the other hand, from the viewpoint of the destruction of the environment, it is an important urgent problem that the enterprises should tackle to reuse the used chemicals, reduce waste generated from the plants and conserve energy of a variety of equipment for the achievement of cost reduction.
Conventionally, as a waste water treatment method for reusing the sludge including unreacted chemicals, there are the methods disclosed in the prior art references of Japanese Patent Laid-Open Publication Nos. HEI 5-39830, HEI 8-197070 and HEI 10-5769. Each of these three waste water treatment methods sends the sludge precipitated in the sedimentation tank back to the reaction tank or the coagulation tank where a stirrer is installed. It is to be noted that the prior art reference of Japanese Patent Laid-Open Publication No. HEI 5-39830 discloses pneumatic stirring in addition to the stirring by means of the stirrer.
The fluorine waste water is basically treated by forming a slightly-soluble calcium fluoride. In this case, for the purpose of reducing the sludge as a water generated from the waste water treatment equipment, there is adopted a method for using calcium carbonate mineral instead of the conventional slaked lime method for using slaked lime. As described above, the slightly-soluble calcium fluoride is formed through the fluorine waste water treatment. The calcium fluoride is not dissolved under the condition of either acid or alkali. Therefore, the calcium agent of slaked lime or calcium carbonate mineral can be reused again and again through circulation or another method to enable the formation of calcium fluoride that acts as the core of floc, so that the reduction of waste and the reduction in amount of use of chemicals are achieved to allow efficient waste water treatment to be achieved.
The above matter will be simply described. First of all, in a first step, the calcium agent of slaked lime or calcium carbonate mineral is added to form slightly-soluble calcium fluoride, by which the fluorine in the waste water is treated to a concentration of about 20 ppm to 40 ppm. Next, in a second step, an inorganic coagulant of polychlorinated aluminum or the like is added to the waste water obtained through the first step, and then a macromolecular coagulant is added to reduce the concentration to the intended concentration. In the above second step, the fluorine concentration cannot be reduced to the intended concentration unless a large amount of inorganic coagulant such as the polychlorinated aluminum or the macromolecular coagulant with respect to the amount of fluorine in the waste water is added. As a result, the unreacted aluminum, macromolecular coagulant and so on precipitate together with the slightly-soluble calcium fluoride to be consequently included in the sludge. Therefore, by sending the sludge including the unreacted aluminum, macromolecular coagulant and so on back to the reaction tank or the coagulation tank, the unreacted aluminum and the unreacted macromolecular coagulant are reused.
As a method for treating the water containing fluorine by means of calcium carbonate, there is the prior art reference of Japanese Patent Laid-Open Publication No. HEI 7-136667. According to this treatment method, the water containing fluorine is made to flow through a plurality of towers containing calcium carbonate, and after the outflow water that has flowed through each tower containing calcium carbonate is aerated, part of the water is made to flow through the immediately preceding tower containing calcium carbonate. By thus aerating the outflow water of each tower containing calcium carbonate, H.sub.2 CO.sub.3 in the outflow water is discharged as a CO.sub.2 gas. By subsequently making the water flow through the same tower containing calcium carbonate, the amount of H.sub.2 CO.sub.3 to flow into the tower containing calcium carbonate can be reduced as far as possible. Consequently, the amount of addition of the alkali agent of ammonia, ammonium fluoride or the like for the prevention of the collapse of the calcium carbonate filler material due to H.sub.2 CO.sub.3 can be reduced.
A method for treating an organic waste water containing fluorine is disclosed in the prior art reference of Japanese Patent Laid-Open Publication No. HEI 5-4090. According to this treatment method, the water-soluble calcium compound of slaked lime, calcium chloride or the like is added to the organic waste water containing fluorine so as to coagulate and precipitate the calcium fluoride. After adjusting pH of the supernatant liquid to 6.5 to 7.0, the water is brought in contact with fixed microorganic pellets to undergo an aeration treatment for the removal of BOD (Biological Oxygen Demand) components. Subsequently, a coagulant is added to precipitate together the microorganisms leaked from the fixed microorganic pellets and the remaining fluorine compound. By thus adjusting the liquid pH to 6.5 to 7.0 and bringing the organic waste water containing fluorine to the fixed microorganic pellets after the coagulo-sedimentation treatment, the calcium is prevented from adhering to the micropores of the fixed microorganic pellets, so that a high rate of removal of fluorine and BOD can be consequently achieved.
Waste water treatment equipment for treating fluorine waste water containing organic matter is disclosed in the prior art reference of Japanese Patent Laid-Open Publication No. HEI 9-174081. As shown in FIG. 14, this waste water treatment equipment has a first water tank 1 having an upper portion 1A and a lower portion 1B. Then, the upper portion 1A is provided with an air diffusion pipe 2 and containing a granular calcium carbonate mineral 3. This calcium carbonate mineral 3 is made to strongly flow by the aeration of the air diffusion pipe 2. The fluorine in the waste water supplied from the downside of the lower portion 1B chemically reacts with the calcium carbonate mineral 3 and becomes calcium fluoride to treat the fluorine in the waste water. On the other hand, microorganisms propagate on the surface of the calcium carbonate mineral 3 that has subsided from the upper portion 1A to the lower portion 1B of the first water tank 1, thereby biologically treating the organic matter in the waste water. Thus, the calcium carbonate mineral 3 that has moved to the lowermost portion of the lower portion 1B is moved toward the upper portion 1A while being mixed with the waste water by an air lift pump 4 so as to chemically treat the fluorine in the waste water in the upper portion 1A. Then, the inorganic sludge and the biotic sludge that are comprised mainly of calcium fluoride and generated in the first water tank 1 are subjected to a coagulation treatment by polychlorinated aluminum added as an inorganic coagulant. Thereafter, the sludge is moved to a third water tank 6 so as to be separated into sludge in the form of a precipitate and a treated water in the form of a supernatant liquid.
Waste water treatment equipment capable of concurrently treating waste water and exhaust gas containing fluorine and surface active agent is disclosed in the prior art reference of Japanese Patent Laid-Open Publication No. HEI 8-57498. As shown in FIG. 15, in this waste water treatment equipment, a calcium carbonate mineral 12 is placed in a lower portion of a first reaction regulation tank 11, while a calcium carbonate mineral 12 and a plastic filler 13 are placed in an upper portion. A waste water containing fluorine and surface active agent is firstly aerated and stirred in the lower portion of the first reaction regulation tank 11, sprinkled in the upper portion of the first reaction regulation tank 11, aerated and stirred in the lower portion of a second reaction regulation tank 14, sprinkled in the upper portion of the second reaction regulation tank 14, subjected to coagulation with coagulant added to a reaction coagulation tank 15 and then subjected to solid-liquid separation into sludge as a precipitate and supernatant liquid in a sedimentation tank 16. It is to be noted that the sludge (mixed sludge including inorganic sludge, organic sludge and biotic sludge) precipitated in the sedimentation tank 16 is returned to the upper portion of the first reaction regulation tank 11 by a pump 17 and sprinkled so as to be used for the separation treatment into a waste water and an exhaust gas.
FIG. 16 shows waste water treatment equipment for treating fluorine waste water containing surface active agent, phosphor and hydrogen peroxide using calcium carbonate mineral, slaked lime and coagulant (polychlorinated aluminum and macromolecular coagulant). In this waste water treatment equipment, a calcium carbonate mineral 23 is placed in a second water tank 22 for the purpose of reducing the amount of waste generated from the waste water treatment equipment. Then, the calcium carbonate mineral 23 and the fluorine in the waste water are made to react with each other. However, the fluorine concentration in the waste water is not stabilized below 8 ppm, and therefore, the slaked lime, polychlorinated aluminum or macromolecular coagulant serving as the coagulant is excessively added as compared with the amount of fluorine in the waste water. The slaked lime is effective particularly for the treatment of phosphor in the waste water. There are further shown a first water tank 21, a blower 24, a third water tank 25 that serves as a slaked lime reaction tank, a fourth water tank 26 tank that serves as a polychlorinated aluminum coagulation tank, a fifth water tank 27 that serves as a macromolecular coagulant reaction tank, a sixth water tank 28 that serves as a sedimentation tank and a seventh water tank 29 that serves as a condensation tank.
However, the aforementioned prior art method for treating the fluorine in the acid waste water by forming slightly-soluble calcium fluoride has had the following problems. That is, a large amount of inorganic coagulant of polychlorinated aluminum or the like or macromolecular coagulant with respect to the amount of fluorine in the waste water in order to reduce the fluorine concentration to the intended concentration in the second step, and the sludge including the unreacted aluminum, macromolecular coagulant and so on is set back to the reaction tank and the coagulation tank in order to reuse the unreacted aluminum and unreacted macromolecular coagulant, which are generated for the above reasons.
However, the stirring by the generic stirrer in the reaction tank and the coagulation tank has a lack of capacity for decomposing the sludge as well as a short retention time. This also leads to the problem that the unreacted chemicals cannot completely be regenerated as calcium ions and aluminum ions from the sludge, resulting in low regeneration efficiency.
As described hereinabove, the waste water treatment method disclosed in the prior art reference of Japanese Patent Laid-Open Publication No. HEI 5-39830 executes agitation by the agitator and air by sending the sludge precipitated in the sedimentation tank back to the reaction tank. However, due to the short retention time in the reaction tank, the method still has a lack of capacity for decomposing the sludge, and this leads to the problem that the regeneration efficiency is low. Further, the waste water treatment method disclosed in the prior art reference of Japanese Patent Laid-Open Publication No. HEI 10-5769 similarly sends the sludge back to the coagulation tank having a short retention time instead of the reaction tank. Therefore, the sludge is not sufficiently decomposed, and this leads to the problem that the regeneration efficiency is low.
As shown in FIG. 14, the waste water treatment equipment disclosed in the prior art reference of Japanese Patent Laid-Open Publication No. HEI 9-174081 does not execute at all the return of the sludge that has been coagulated or precipitated in the second water tank 5 that serves as a coagulation tank or the third water tank 6 that serves as a sedimentation tank to the first water tank for the reuse of the sludge, meaning that the sludge is disposed as a waste with the unreacted chemicals included. Therefore, the chemicals to be used and the waste cannot be reduced at all.
In the waste water treatment equipment disclosed in the prior art reference of Japanese Patent Laid-Open Publication No. HEI 8-57498 shown in FIG. 15, the calcium carbonate mineral 12 is fixed. Therefore, after a long time of operation, the calcium fluoride generated as a consequence of the reaction of the calcium ions released from the calcium carbonate mineral 12 with fluorine accumulates in the gaps and clogs up, consequently worsening the circulation of the waste water, to a problem.
In the case of the waste water treatment equipment for treating the fluorine waste water containing surface active agent, phosphor and hydrogen peroxide by using calcium carbonate mineral, slaked lime and coagulant (polychlorinated aluminum and macromolecular coagulant) as shown in FIG. 16, there are the following problems.
That is, in the second water tank 22, to release calcium from the calcium carbonate mineral 23 by acid and make the calcium efficiently react with the fluorine in the fluorine waste water containing surface active agent, phosphor and hydrogen peroxide, two blowers 24 are provided for aeration stirring at a rate of about 120 m.sup.3 /day per 1 m.sup.3. However, if the two blowers 24 are operated, the charge for electricity increases to cause the problem that the running cost increases. However, if the aeration stirring is operated at a rate of 60 m.sup.3 /day per 1 m.sup.3, then the efficiency of reaction of the waste water with the calcium carbonate mineral 23 is low, and this leads to the problem that the fluorine concentration in a separation chamber 30 at the exit of the second water tank 22 consequently is achieved by one half or lower than one half the target value (for example, assuming that the target value of the fluorine concentration is 20 ppm, then the fluorine concentration in the separation chamber 30 becomes equal to or higher than 40 ppm).
Furthermore, it is required to dehydrate the sludge including the unreacted chemicals (i.e., sludge including hydroxides of aluminum and calcium). Therefore, a plurality of filter press units 31 and 32 are needed and the operation time thereof is long, for which the charge for electricity also increases to cause the problem that the running cost increases.
Furthermore, the surface active agent that serves as an organic matter in the waste water is somewhat decomposed by the aerobic microorganisms in the second water tank 22. However, for the reason that the microorganic concentration in the second water tank 22 is low and the biodegradability of the surface active agent is bad, there is the problem that the surface active agent cannot sufficiently be decomposed and removed. Specifically, the extraction ratio of the surface active agent in the second water tank 22 is intended to be 50% or higher, however, the current practice is unable to secure 50%. The hydrogen peroxide in the waste water is more or less decomposed by the anaerobic microorganisms propagating in the third water tank 25 through the sixth water tank 28, whereas the extraction ratio of the hydrogen peroxide is not greater than 50%.
The extraction ratio of phosphor in the waste water can be secured at a rate of not smaller than 90% since the slaked lime is added to the third water tank 25. However, in order to secure the extraction ratio of 90%, there is the problem that the slaked lime must be excessively added more than is needed for phosphor in the waste water. In this case, the reason why the slaked lime is excessively added is that the unreacted slaked lime flows out of the third water tank 25 to the fourth water tank 26 since the slaked lime tends to easily flow without sinking when added to the third water tank 25 and the reaction time continues shortly within one hour. The same thing can be said for not only the slaked lime but also the polychlorinated aluminum added to the fourth water tank 26 and the macromolecular coagulant added to the fifth water tank 27. Therefore, as a result, the unreacted slaked lime sludge and the unreacted coagulant sludge (i.e., hydroxide sludge of calcium hydroxide, aluminum hydroxide and so on) attributed to the slaked lime, polychlorinated aluminum and macromolecular coagulant exist in the sludge precipitated in the sixth water tank 28, consequently causing an increase in the amount of generated sludge.
That is, in the waste water treatment equipment shown in FIG. 16, the amount of generated sludge (amount of sludge) is reduced as compared with the waste water treatment method for executing treatment with the slaked lime and coagulant without using the calcium carbonate mineral 23. However, the unreacted slaked lime and the unreacted coagulant are left in the sludge, meaning that this waste water treatment method is not most appropriate in the current age of waste reduction. Therefore, the reuse of the unreacted slaked lime and the unreacted coagulant emerges as a big problem.